Clock signals are typically used for synchronization in data processing systems (e.g., handheld personal data assistants and computers, such as palm computers, desktop personal computers or notebook computers). One or more clock signals may be used for the synchronization of the operations in various components in a data processing system and for the synchronization of the operations in the digital circuits within the various components of the data processing system. For example, the central processing unit (CPU) of a computer may operate at one clock frequency; and, the random access memory (RAM) of the computer may operate at another clock frequency. Typically, different clock signals in a data processing system are derived from a single source. The relation between the different clock signals is typically fixed so that the operations in different components can be easily synchronized.
Clock signals at a fixed frequency generate and radiate electromagnetic interference (EMI) that has spectral components peaked at the harmonics of the fundamental frequency of the clock circuit. Government regulations (e.g., FCC regulations in the United States) specify the maximum allowable emissions for electronic products. Thus, data processing systems are designed to comply with these EMI regulations.
The EMI from the clock signals increases as the clock speed and power increases. To comply with the EMI regulations, EMI from the clock signals are reduced through a number of design considerations. For example, clock lines may be routed carefully to minimize loops and other potential radiating structures. Shielding may be used to contain the EMI from the clock lines. EMI reduction can also be achieved through slowly modulating the clock frequency so that the spectral components of the EMI from the clock signal are spread out in the emission spectrum. Thus, the spread spectrum modulated clock signal does not get concentrated at the harmonics of the fundamental frequency.
A spread spectrum clock is one in which the center frequency of the clock is modulated slowly (e.g., with one complete modulation cycle every 33 μs or so). Various time domain “profiles” of the frequency deviation can be used. Typically used profiles are sinusoidal, triangular, or piecewise polynomial. The degree of EMI reduction achieved through a spread spectrum modulation is typically a function of the specific profile used. For example, a triangular profile typically provides a greater degree of EMI attenuation than a sinusoidal profile.